1. Field of the Invention
The present invention relates to an irradiation control device, and in particular to an irradiation control device used for an imaging device or the like which monitors or takes an image of a person, a character or the like in the nighttime or in a dark place.
2. Description of the Related Art
In the nighttime or in a dark place, a person, a character or the like is recognized by monitoring same by using a camera with irradiation and processing the output image of the camera. In such a case, a system using near-infrared irradiation by an LED (light-emitting diode) for making the irradiation as unnoticeable as possible has been known.
FIG. 10 shows a schematic diagram of such a system, in which an imaging (image pick-up) device 14 composed of an irradiating portion 12 with an LED 11 and a camera 13 is placed on a utility pole 15 on a road shoulder. When taking an image of or photographing a subject such as a vehicle 16 on a road by the camera 13 of the imaging device 14, the LED 11 of the irradiating portion 12 is lighted so that the image of the subject 16 is acquired by the near-infrared rays.
At this time, a method of shortening a lighting time of the LED 11 as much as possible or the like has been adopted in order to extend the longevity of the LED 11 as long as possible.
Specifically, a lighting time control has been performed as follows:    (1) The LED is lighted at a shutter timing of the camera;    (2) Brightness or luminance of the image taken is detected by image processing, and only when it becomes dark to some extent, the LED 11 is lighted;    (3) Normally, the LED 11 is lighted darkly and a motion is detected by the image processing, so that when a motion change is detected, the output of the LED 11 is increased.
On the other hand, a TV camera irradiation device has been proposed having a semiconductor light emitting device irradiating a subject, and a light emitting time controller which activates the semiconductor light emitting device to emit for a predetermined time including an imaging time in response to an image timing signal from a CCD imaging TV camera and to intermit a light emitting body for a fixed time (see e.g. patent document 1).
Also, a vehicle number recognition device has been proposed which connects a shutter synchronous signal to an irradiation in order to synchronize with a shutter open time of a television camera, sets a pyroelectric sensor, and lights the irradiation only when a vehicle is detected farther than a view of the camera (see e.g. patent document 2).
Furthermore, an image monitoring device has been proposed which has an irradiation projecting control means lighting or extinguishing an irradiation device according to average luminance of an image obtained from a captured image of a monitoring area (see e.g. patent document 3).
<Patent Document 1>
Japanese Patent Application Laid-open No. 5-219447 (Abstract and FIG. 1).
<Patent Document 2>
Japanese Patent Application Laid-open No. 10-261190 (Abstract and FIG. 1).
<Patent Document 3>
Japanese Patent Application Laid-open No. 2000-306684 (Abstract and FIG. 1).
However, since an operating current control of the LED that is a light emitting device has not been performed in the prior art lighting control, as shown in a light quantity change characteristic A of the prior art example of FIG. 11, the emitting light quantity (ordinate) of the LED linearly attenuates for a total energization time (abscissa).
Accordingly, in order to secure at least a minimum light quantity OM1 required for image processing, a subject recognition or the like for a minimum longevity L1 (e.g. two years), an operating current which can provide a light quantity OM2 larger than the light quantity OM1 has to be flowed from the time when the LED was set or mounted.
In other words, it has been required to flow a large operating current from the time when the LED was set or mounted to emit the light quantity OM2, so that deterioration of the LED has been accelerated to shorten its longevity L1.